The present invention relates to a sheet supply device and an image forming apparatus in which each sheet is separated from a bundle of sheets stacked on a sheet placement table and is supplied.
An image forming system such as a photocopier, facsimile and printer, is provided with a plurality of sheet supply units for storing sheets of paper, and the sheets stacked and stored in each of the sheet supply units are separated from one another one by one and are fed out. The sheets of paper fed out are conveyed to an image forming section, whereby images are formed on them.
Generally, a conveying apparatus equipped with a sheet supply unit of roller conveyance method is extensively used as a system of separating and conveying sheets one by one. The surface of the roller conveyance system is made of such an elastic body as a rubber characterized by high friction coefficient. Sheets are separated from one another one by one by the friction force between the sheet and the surface of the roller in contact with the sheet, and are conveyed.
In the meantime, an image forming system using electrophotographic process has come into use in the field of quick printing including POD (print on-demand) in recent years, and it is essential to meet a great variety of needs regarding the image quality and recording medium. Particularly to meet the requirements with respect to the recording medium, gloss coated paper, quality paper, printed paper or recycled paper is employed in some cases.
To improve the surface gloss, the gloss coated paper is coated with resin or the like. This enhances the level of smoothness on the surface. When such gloss coated paper is used, a required frictional force cannot be obtained between the paper and roller surface in a roller conveyance type conveyance device, and sufficient sheet supply performances cannot be ensured. Further, when printed paper is used, the paper releasing agent (powder) attached to paper is transferred to the roller surface. When recycled paper is used, powder of paper is transferred to the roller surface. In both cases, this results in reduction in frictional force, and stable and sufficient sheet supply performances cannot be ensured for a long time. This problem has been left unsolved.
One of the solutions to the aforementioned problem is proposed by an air conveyance device provided with an air conveyance type sheet supply unit (Japanese Unexamined Patent Application Publication No. 2003-171024 and Japanese Unexamined Patent Application Publication No. 2007-276910). The air conveyance device blows air from the surroundings to the top of the sheet bundle stacked and stored in the sheet supply unit so that the sheet of the topmost surface can be easily separated from others by sending air between the sheets. Then the sheet of the topmost surface is sucked by the conveyance belt, whereby sheets are conveyed one by one. The sheet is sucked by the conveyance belt by means of the pressure difference of the air blow produced by the rotation drive of the suction fan. Such an air conveyance device is so structured as to meet the requirements of a great variety of recording media.
To ensure that the sheet of the topmost surface of the sheet bundle stored in the sheet supply unit is sucked by the conveyance belt and is conveyed, the distance from the conveyance belt must be kept within a predetermined range by detecting the position of the topmost surface of the sheet bundle and raising the elevating stand carrying the sheet bundle in conformity to the detected value.
In this proposed method, the position of the top surface of the sheet bundle is detected by the top surface detecting section of the sheet bundle wherein a contact member is brought in contact with the top surface of the sheet bundle, and the movement of the contact member displaced in conformity to a change in the position of the top surface of the sheet bundle is read by an optical sensor.
However, in the air conveyance device disclosed in the Japanese Unexamined Patent Application Publication No. 2003-171024 and Japanese Unexamined Patent Application Publication No. 2007-276910, the top surface is detected by the detecting section that employs a contact member. This arrangement brings the contact member in contact with the sheet bundle. This may damage the sheet when a delicate sheet such as thin paper or computer output sheet is used. Further, in the air conveyance type sheet supply unit, air must be blown to the top surface of the sheet bundle to separate or isolate sheets. The air blow may be interfered by the contact member, with the result that sheets cannot be separated or sorted out.
To solve such a problem, a non-contact sensor could be used to detect the top surface of the sheet bundle. However, during the sheet supply, a plurality of sheets are floated at different positions on the top portion of the sheet bundle by the air blow for the purpose of separation or sorting of sheets. This leads to a failure in the accurate detection of the uppermost point of the sheet bundle in the non-floated state. The uppermost point of the non-floated sheet bundle cannot be maintained at the predetermined position (height). Such problems have been left unsolved in the conventional art.
In view of the prior art problems described above, it is an object of the present invention to provide a sheet supply device in which the uppermost point of the non-floated sheet bundle is always kept at a predetermined position during the sheet supply, without the internally stored sheets being damaged, although the aforementioned apparatus is an air conveyance type sheet supply device.